Method of treating electrolytic coatings on magnesium and its alloys



Patented July 8, 1941 METHOD OF TREATING ELECTROLYTIC COATINGS ON MAGNESIUM AND ITS ALLOYS Robert W. Buzzard, Kensington, Md.

No Drawing.

Original application June 29, i

1937, Serial No. 150,932. Divided and this application April 26, 1938, Serial No. 204,388

(Granted under the act of March 3, 1883, as

I amended April 80, 1928; 370 0. G. 757) 4 Claims.

The invention disclosed and claimed herein is a division of my copending application, Serial No. 150,932, filed June 29, 1937, now Patent No. 2,203,670 of June 11, 1940.

This invention relates to the treatment of electrolytically formed coatings on magnesium and its alloys and one of its objects is to increase the durability and the protective characteristics of the film by sealing the coating.

The method specifically relates to the treating of electrolytic coatings on magnesium and its alloys with a solution in the presence of heat so "as to form a seal that not only increases the. durability of the article coated to wear but also improves its resistance to corrosion.

The article to be treated in this manner, which has been coated electrolytically, is washed free of solution, providing the electrolytic bath is basically difierent from the solution used in my sealing method and is then immersed in a hot solution of sealing compoundsppreferably at or change in color, whereas the unsealed coating above the boiling point of the solution, but below the fusion point of the metal.

The sealing solution should contain a corrosion inhibiting ion such as either a chromate or dichromate anda second salt as a reacting ion such as manganese -or chromium or borate, phosphate, fluoride or a combination of any of the said second salts. The concentration of the salts is limited only by their solubility, but may be varied to conform with timeof "application and quality of seal. Good seals of coated metal have been obtained by subjecting the coatings to treatment-for period-s varyingfrom five minutes to one hour. i

Specifically, as an example, if an anodic coating be formed on magnesium or its alloys in a bath of 10 per cent dichromate and a 2 per cent monoba-sic phosphate and the coating has been washed free of anodizing solution, its corrosion resistance will be greatly improved if it is immersed in a solution of dichromate and manganous sulphate at boiling temperature of water or above. The sealing solution may be variedto contain a trivalent chromium salt as chromiuin-acetate or sulphate or fluoride. A chromate may be introduced or substituted in the solution or a phosphate may be substituted or. added when the solution is of such composition not to cause detrimental intersolution reactions.

It should be understood that the protection obtained bymy treatment is dependent on .the

presence of an electrolytic coating and is not a prime surface treatment.

showed a radical change'in color.

As further examples of compositions which will satisfactorily'seal electrolytic coatings on magnesimn, the following are cited:

(a) 1 to 30 percent dichromate i l to 30 percent borate (b) '10 percent sodium dichromate 5 percent sodium borate (c) 5 to 10 percent ammonium dichromate 5 to 10 percent sodium borate '5 to 10 percent manganous sulphate (d) 8 to 1 2 percent dichromate Sto 7 percent sodium borate 'A double dip may also be used such as firstimmersing the electrolytically coated article in a solution of dichromate and phosphates followed by an immersion in a. solution of manganese and chromium sulphate. Ammonium and other salts may-be substituted for the sodium and potassium salts.

The pH of the sealing'solutions described herein may be on the acid or alkaline side, depending upon the relative proportions of the salts employed, and the characteristic pH of solutions of the individual salts.

Theabove mentioned substances used in the magnesium treatments are mentioned by way of illustration and not by limitation, since the invention comprehends treatments of the character indicated broadly above'and is not to be limited except by the appended claims.

The invention described herein may be manufactured or used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What I claim is:

1. The method of producing corrosion resistant coatings on magnesium and magnesium alloys in which magnesium predominates comprising forming on the metal an anodic coating produced by reaction with magnesium and subjecting said coating to a non-electrolytic treatment in an aqueous solution containing a water soluble borate and a water soluble compound selected from group consisting of chromate and dichromate.

2. The method of producing corrosion resistuant coatings on magnesium and magnesium alloys in which magnesium predominates comprising forming on the metal an anodic coating produced by reaction with magnesium and subjectin'g said coating to a non-electrolytic treatment in an aqueous solution containing sodium borate and a water soluble compound selected from group consisting of chroma-to and dichromata,

3. A method of producing corrosion-resistant magnesium or magnesium alloys comprising forming an anodic coating produced by reaction with magnesium on the metal and subjecting the coating to a non-electrolytic treatment in an aqueous solution containing 8 to 12 percent water soluble dichrom-ate and 3 to 7 percent sodium borate, and maintaining the ten'rperatureof the solution during treatment substantially equivalent to the boiling point of the solution.

4. The method of producing corrosion resistant coatings on magnesium or alloys thereof in which magnesium predominates comprising forming on the metal an anodic coating produced by reaction with magnesium, and subjecting sail coating to a non-electrolytic treatment in an aqueous solution containing a water soluble borate and a water soluble compound selected from the group consisting of the chromates and dichromates, the pH of such solution being on the 

